text:
why do all planes hold position when Air Force 1 is taxiing?
Why does everyone hold in position when Air Force One is just taxiing? This can cause a delay that could cascade to other airports.

Because the PotUS' security detail is paranoid.

Since AF1 movements are generally planned, airlines do have a chance to take steps to minimize delays - They probably still occur, but can be reduced.

No doubt security has a lot to do with it. With only one vehicle supposed to move on the airport premises, everything else that moves can be easily spotted and classified as a potential threat, and resources dispatched to handle it.
If there are dozens of other aircraft and hundreds of cars and trucks moving around, this gets a lot harder.


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Does aeronautical experience ever expire?
Do the hours of flying with instructor or solo ever expire? I have my 50 (yes, 50) year old pilot's Flight Log with CFI entries for 20 hours dual and 1 solo. Do I have to start over, or would some of them count for PPL license?

As far as the FAA is concerned, they do not expire.
As far as any (reputable) flight school goes, they will probably require a full, recent minimum 40 hours of training before they sign you off for the practical exam.
The good news is that your old training, no matter how rusty, probably means you'll pick up the basics quite quickly, and probably solo (again) faster than the average, and may very well be ready for your checkride close to 40 hours.  (The national average is a checkride around 50 to 60 hours.)

My personal requirement is at least 40 recent hours. I was surprised there's no expiration.

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text:
How much thrust is gained from placing a propeller in a duct?
Assuming that the power and diameter are the same, how much extra thrust can be gained by placing a propeller in a duct? How does it influence the RPM?
To clarify: take any prop, place it in a well-designed duct, what can we expect? I expect higher RPM for same power, but also duct enhanced thrust. An approximation based on sources would be great.

That "duplicate" question basically says, "maybe even twice" the thrust, which is more an intro to, rather than an answer to, this question.

@mins I don't think he does mean "wing lift." How would a ducted prop produce any major effect on the wings. When he says "lift" I believe he is referring to thrust. Which makes it a dupe of the other question except that it's asking for some more specific quantities.

@whiteknight can you confirm if you are referring to thrust, not lift? (well technically the thrust is the "lift" of the prop blades, but thrust is less confusing)

@TomMcW: I've reverted to the OP's version. BTW I haven't seen the tag 'helicopter'.

I do not understand your question. You want to know which is the power of a propeller in a wind tunnel that completely encloses it or just of a propeller mounted on a plane with a structure that encloses the prop?

This question is difficult to accurately answer.  For example, are you optimizing the ducted prop, or is it the same airfoil used in the unducted version?  Also, RPM is normally a function of the prop airfoil, and other things, so that implies that you are considering the same airfoil, whether ducted or not.  Then if I can pick the airfoil, I can change the RPM behavior, whether ducted or not.  Or are you simply looking for a rule of thumb?  What is the relative wind of the prop? And so on. Tell us more, and you will get more info.

A well designed duct...for what?  What is the application, what are the bounding parameters?  I posted an answer, and while it addresses thrust efficiency issues it does not address RPM.  RPM is influenced by a number of parameters, and there is no definition as to what the assumptions might be.

Relative to MAV, as opposed to large scale rotors:

Compared to the baseline open rotor, the shrouded rotors showed increases in thrust by up to 94%, at the same power consumption, or reductions in power by up to 62% at the same thrust.
At rotor collectives above 10°, all shrouded-rotor models showed improvements in performance over the isolated (‘open’) rotor — that is, higher thrusts and lower power requirements. At the same power consumption, increases in thrust over the open rotor by up to 94% were observed, or, conversely, up to 49% reductions in power at the same thrust, for the condition where the open and shrouded rotors are required to have the same tip speed. If, instead, the rotor disk areas are required to be the same, then up to 90% increases in thrust at the same power.
The effects of changing any one of the four parameters (tip, rlip, d, Ld) on the shrouded-rotors’ performance became more pronounced as the other three
parameters were changed to degrade the performance. It is therefore not possible to categorically state that any one particular parameter has a greater
effect on performance than any other, since the amount of influence of any
parameter depends on the values of the other parameters. The exception to
this seems to be the lip radius, changes in which had almost same effect no
258 matter what the values of the other parameters are.
Thus, in general, it would seem that changing the value of any shroud parameter so as to improve performance results in less sensitivity of the performance to changes in any of the other parameters; similarly, improving the efficiency of the rotor itself would seem to result in less of a performance benefit from the presence of a shroud.

Extracted from:
HOVER AND WIND-TUNNEL TESTING
OF SHROUDED ROTORS FOR IMPROVED
MICRO AIR VEHICLE DESIGN
Jason L. Pereira
Doctor of Philosophy, 2008
Dissertation directed by: Professor Inderjit Chopra
Department of Aerospace Engineering University of Maryland College Park
Rotor parameters including tip gaps, tilt and relative wind, collective, and motor characteristics will influence rotor RPM, and are indirectly addressed in Pereira's dissertation.  Again, there are very broad assumptions in the OP question, but Pereira substantiates generalized efficiency improvements for specific configurations.

Could you please elaborate on the test setup?  It would be of worth to the question, I think, to describe the orientation of the shrouded rotor in the wind tunnel.
Also, nice pick...I should've thought to check this.  I walk past the results of this guy's thesis work every day...

He addresses several configurations, and they cannot be done justice in an answer here.  I suggest you read the dissertation.  It is widely available, and he has numerous diagrams, describing hover, translational flight, etc.

Acknowledged.  However, since the OP seems to be asking about a shrouded propeller, it's not wholly clear to me that the answer pertains specifically to that because "rotors" normally refer to propellers in edgewise flight or hover.  Could you please clarify what configuration the power/thrust benefits you state pertain to?

My familiarity with the cited dissertation is general (it's a couple of hundred pages long) but he does cover what happens when the relative wind is changed (as in forward flight) and at different angles (hover, translational, cruise). I believe his quotes statement is for hover, but if you read the paper it also applies to other relative angles and wind (airspeed in a vector).  The author does not make a generalized statement akin to what the OP asks, and as I have said before, there are many assumptions involved in generalizing the results, so I decline to do so.

For example, "take any prop and place it in a well-designed duct" fails to recognize that rotors, props, spinning wings or whatever they are called work as a system in a duct, and the combined system has a performance in a certain environment. The assumptions are too broad, and this poorly bounded question keeps getting rewritten.

Got it.  Entirely fair.  I'll see if I can have a look through the dissertation at some point...sounds interesting.

@ymb1, I realize that you are trying to clarify the question and get effective answers.  But when the question wording changes, etc. it makes answering a moving target.  I wasn't trying to roast anyone, just venting a little frustration (grin) which was probably due to nothing here.

That "any prop" part is the killer.  Most people get less thrust from trying that:

My experiments suggest only a a max of 20% improvement is possible:

It should spin slower (because more of the prop is producing lift, instead of stuck in the vortex at the tips) for the same power.
The "well designed" part of your question is also non-trivial - there's no such thing if you want the prop to perform optimally across a range of airspeeds - the duct has to be designed for a specific operating condition, so it will always make things worse at "off conditions" - which will usually be more worse than the prop without the duct.
The entire point of the duct is to reduce tip vortices, and the entire point of the tip region of "any propeller" is also to reduce vortices, so using both at once is not going to work properly, if at all (see video above - in real life, it does NOT work).  The tips of a duct fan need to be totally different shape to the tips of "any propeller".
I question the accuracy of the paper mentioned: he shows that reduced diffuser angle improves thrust (p. 51 - but totally fails to consider Bernoulli for that, and never even tried angles less than 0 degrees), and his results and diagrams are almost all based on large diffuser angles, which is the opposite to Bernoulli, common sense, and modern practice.
In my relatively vast experience in this field, almost all CFD models and predictions are false (papers are usually written by students, none of whom understand modern ML techniques, especially the "time traveller conundrum" as it applies to CFD validation).  Long story short - they do not realize that they're "training" their math to match the observations [almost always of poor experiments, and always of a tiny sample], instead of what the math is supposed to be - a prediction, for more than just the sample observed.  Bottom line - if it's not something real that was measured, you can't trust the numbers.

I guess anything about "vorticies" will pique the interest of many a young engineer.  Props generate lift as wings do. Notice many of them are high aspect for the same reasons that wings are, backing off to chunky low AR only when Mach becomes an issue.  With shrouds, benefits of prop drag reduction are offset by additional weight and, as airspeed increases, more drag (from the shroud itself).

as you say The tips of a duct fan need to be totally different shape to the tips of "any propeller", so, is there a better test. that does a better comparison using a different propeller shape for the duct than the same shape of open prop?

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Why does the US Marines continue to fly the AH-1 Cobra, retired by the US Army in the late 90s?
My father flew Bell AH-1 Cobra for the United States Army in the late 1970s and a young man in my daughter's USN winging class (July 2017) was being assigned to Camp Pendleton to fly Cobra's for the United States Marines.  From talking to my father and reviewing the Cobra wiki page, it appears that the Army retired the Cobra in the early 1990s.  However, it seems the Marines have continued to fly the Cobra, though AFAICT, they seem to have moved to the twin Super Cobra:

I am aware that other attack helicopters followed the Cobra and that the Army adopted the AH-64 in the mid 1980s.
What's not clear is why the Army retired the Cobra but the Marines continued to fly it.  Or, perhaps another way to ask this question would be, why does the USMC favor the Cobra over the newer AH-64?

It might be obvious but money definitely play a large factor here.

Or, you could rephrase the question as "Why did the army stop using what is clearly a perfectly good helicopter?"

Well this ain't your father's Cobra.  Literally.
The current incarnation of the Huey cobra for the Marine Corps, which is called the AH-1Z Viper,  sometimes referred to as the King Cobra continues to be the backbone of the Marines attack helicopter force for several reasons.
Like all Huey cobra models, the AH-1Z and the USMC current utility helicopter, the UH-1Y Venom, make use of a common power plant, transmission and rotor set, tailboom and avionics architecture, saving quite a bit on spare parts.  The USMC quotes an 86% commonality between the UH-1Y and AH-1Z airframes.  And, as both the UH-1Y and the AH-1Z constitute a single acquisitions program, it saves a lot of time and costs trying to begin another weapons procurement program for the Corps.  This is also the reason the Huey Cobra even came into existence in the first place.
As helicopters have hit the upper limit in terms of performance, the procurement of a brand new airframe over an existing design doesn't offer many advantages and costs considerably more.  Simply upgrade the engines, add in a modular avionics bay with modern infrastructure facilitating upgrades to modern mission systems and modern weapons, and modern defensive countermeasures systems, and you have help that's as good or better than any existing attack platform out there.
The AH-1Z is comparable to the AH-64 Apache weapons system but remains slightly faster, carries a greater payload and has a 33% greater range than the Apache, all preferred attributes for shipboard operations.

The Marine Corps could not use the same variant of Apache that the US Army uses, it would have to be adapted into a Marine version and the numbers would simply not be there to make it an economical option. From the wiki:

In the early 1980s, the U.S. Marine Corps sought a new navalized helicopter, but it was denied funding to buy the AH-64 Apache by Congress in 1981...
..Developing a marine version of the Apache would have been expensive and it was likely that the Marine Corps would be its only customer.2 They instead signed a contract for upgrading AH-1Ws into AH-1Zs

But that is not to say that the latest versions of Cobra are outdated. The AH-1 started out as an attack version of the Huey, but has been updated and re-engineered many times, utilising the experience gained to a maximum. The AH-1Z has a 4-blade bearingless main rotor, which gives even tighter airframe-rotor coupling and manoeuvrability than the offset-hinge rotor can provide the Apache with. The original twin blade teetering rotor design can have a problem with mast bumping when zero or negative g is reached in a manoeuvre - teetering rotor pilots are taught to avoid mast bumping, however having to restrict an attack helicopter in manoeuvring is sub-optimal. The bearingless rotor is not bothered by mast bumping at all.

Updating a known airframe with new systems, engines, weapons systems etc. is not a method to be knocked. If the existing airframe can still support the updated weapons systems, there is no problem to solve. A new platform does not always make sense per se, as this 2010 article in The Economist mentions:

Mr Pugh also identified another intriguing trend: the race for bigger, better weapons is fiercest in peacetime but tends to fall once war actually breaks out. At that point, he argues, quantity takes precedence over quality.

The Marine Corps appears as the ultimate in efficiency, first to arrive, swiftest in action, get the job done with the least amount of fuss, and use the tools that are available which must be reliable above all else. That is awesome.

With all due respect to the USMC (and believe me, I have nothing but the utmost respect for the men/women in the field), this article might disagree with your last paragraph.

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How would an IACRA suspension affect future applications?
An interesting question over on the new UAV.SE site asks

Is it possible to get a FAA Part 107 drone license if one has had a pilot license revoked or suspended?

The answer appears to be that it's not immediately fatal to the application (they would undoubtedly warn you not to bother if you've had a suspension or revocation), but they do ask about it.
In general, how does an IACRA suspension and/or revocation affect future applications of any type?


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Why would a flight no longer considered airworthy be redirected like this?
I ran across this tweet covering the saga of Smartwings 1201, a 737 MAX 8 that was apparently redirected from Prague to Ankara after the MAX was grounded by the EU.

(flightaware.com)
It seems like an odd decision to do this. The plane had to divert and apparently spent quite a long time in circling before it was allowed to land in Turkey. Why was it not allowed to land at Prague as scheduled and be grounded there?

Notice that most of the flight path is over water. If the worst were to happen and it was to crash, there would be minimum damage to those on the ground. Of course, it would mean a significantly reduced chance of survival for those on board...

@FreeMan Crash? Just because it is Boieng 737MAX it does not mean it should crash every minute! They are not THAT dangerous.

@DavidRicherby Well it is not a widebody and they were flying for some time already, but you are right, the difference between MLW and ZFW is not that big. Depends on the load. But no news mention it.

The oddest decision was the decision by the EU not to allow already airborne flights to continue to their destination, I would say.

@VladimirF I said "If the worst were to happen..." A crash is the worst that would happen to any flight. I agree with you 100%, just because it's a 7M8 doesn't mean it will crash, but just about every CAA in the world is treating it that way at the moment.

@FreeMan, India banned the Max with enough notice time to avoid this nonsense.

@DavidRicherby Fuel dumping isn't normally done unless the aircraft is overweight for landing or there is an emergency where they suspect a fire may ensue upon landing (such as stuck landing gear or something like that.) The reason the 737 has no fuel dumping ability is that its max landing weight isn't much less than its maximum takeoff weight. It would certainly not have been overweight for landing by that point in the flight. More likely, they were trying to figure out where on Earth they should go in light of the EASA decision.

Turn off the MCAS and the plane is perfectly airworthy, it might just be more susceptible to stall if the pilots let the nose pitch up too much.

@CrossRoads Not so sure about that.. There have been a lot of sentiments to this effect recently, as if you just switch off the pesky thing and you can then fly the plane like a Sopwith Camel. Apparently that's not the case. Without MCAS, the 737 Max becomes dangerously unstable; accelerate and the nose pitches up, cut the thrust and it tucks. So switching it off is not really an option unless in extremis.

That's the first time I'd heard "dangerously unstable" used.  I was reading this article earlier  https://www.avweb.com/blogs/insider/MAX-Grounded-Now-what-232412-1.html  and the last paragraph stands out: "A friend of mine is a training captain for a major airline and was in town last month. We had a long conversation about this topic and he pointed out that some pilots have just enough knowledge of the panel and systems to get by. And that may not always be enough. Flying with automation is the job, these days."  Maybe that was similar  in these 2 cases.

There could be a lot of reasons for this...

EU closed airspace to 737's MAX 8's on March 12
They needed to go into a holding pattern until ATC figured out where to put them
They needed to be in the holding pattern until they could get a landing slot
They were redirected to an airport that had a maintenance facility that the airline uses
They redirected to an airport with code-share partners so they could rebook passengers without a major fee

Edit I'm not sure what is going on with FlightRadar24, but it shows that the plane continued to Prague the next day. It looks like they landed in Ankara then continued on to Prague. I'm not sure if the flight to Prague was just a repositioning flight, or if it had passengers.
Edit 2 Turkey subsequently (after this flight) also closed airspace to 737 MAX 8's. The EU closure allows for ferry flights, which are flights without passengers on board. The flight from Ankara to Prague was just a positioning flight so that the aircraft could be serviced when it came time to implement a fix from Boeing.

I don't get your first entry. If Turkey has closed their airspace, how come they were redirected to land there?

@pipe They may have allowed the flight to land because it didn't have the fuel for a more appropriate diversion. I'm prettty sure the flight out of Ankara was a ferry flight to Prague.

@pipe pure speculation but it maybe that the airline was able to eventually negotiate permission to enter Turkish airspace as an exception to the ban.

I guess even if a country bans certain planes from their airspace, they surely would allow them to land. If Turkey and neighbouring countries imposed the ban at the same time, the plane theoretically would have nowhere to go.

I made a small edit -- they landed in Ankara at about 2320 and then continued to Prague the following afternoon. That must have been a repositioning flight, since passenger operations are forbidden. The directive banning passenger operations specifically allows for a ferry flight of up to three legs to a maintenance base. (I didn't want to edit all of that in, as it changes the meaning quite a bit.)

@pipe All planes that are in flight land eventually. Mostly, it's better to allow them to land somewhere in a controlled manner, and sort out the paperwork and legal implications later. For the specific question, the airline may not have wanted to land the plane somewhere where it could not be moved until after the technical issues have been sorted out one way or another - parking a plane at an airport for an indefinite period costs money!

@alephzero The original intent of the Airline was definitely to get to Prague. A different flight of the same airline from southwest was trying to get to Italy, as they seemed to be accepting flights already airborne, but this window closed as well https://aviation.stackexchange.com/questions/61040/what-happens-to-an-airborne-plane-when-its-type-is-grounded-by-an-authority/61044#comment159715_61044 The EASA just refused to accept flights with a valid flightplan mid-air without any notice period.

@DohnJoe a new exciting take on "the terminal"

This is just because of the way EASA treated B737 MAX grounding. They just stopped accepting flights with these aircraft  into the EU airspace even for already airborne flights with valid flight plans.
For this company two flights were involved. On from Cape Verde ended up in Tunisia and one from Dubai in Ankara. Both of them were originally hoping to get to the EU airspace until the perceived misunderstanding clears - because flights already airborne and with valid flight plans should be allowed to finished their flights, right, that sounds logical ... not to EASA...
So these aircraft had to land outside EU, get to PAX to the hotels, fly other types of airplanes for them and ferry the MAXes empty home to LKPR.
Other airplanes of the same company became stranded out of EU because they were doing flights between two out-of-EU destinations at the time of the ban and had to wait for a day to be allowed to ferry home.

yeah, several flights ended up circling inside EASA airspace before being directed to the nearest airport with facilities to handle them and their passengers.

There is no evidence that EASA refused Smart Wings QS 1201 to enter EU airspace. The EASA AD does not make any statement about airborne aircraft. Stopping operation mid-air is no option, and a last minute diversion does not contribute to the safety of the passengers.
It is more plausible that the airline had a hard time to decide what to do with this flight, considering all implications including but not limited to legal, operational, commercial and reputational aspects. The internal decision process might have looked similar to the flight track as shown in the question.  ;)
Please see the answer of Paul Saccani in Who decided that the Boeing 737 MAX planes that were airborne when the grounding was issued cannot enter and land in EU air space?

In the case of the EASA directive regarding the B737 MAX: all planes have to be grounded and those in flight weren't allowed to enter European Airspace. Therefore, the plane is trying to figure out where to land.

The directive only says "do not operate". No word about landing or entering. It explicitly allows a ferry flight.

@bogl I think flying with fare paying passengers would fall foul of do not operate.

@Notts90 The directive does not  state the urgency. Throwing the pax out of the door mid flight is no solution, and there is no reason why a diverted landing would be safer than the scheduled one. Common sense would strongly suggest to finish ongoing flights.

@bogl I like you’re optimism regarding common sense! I think do not operate does imply a sense of urgency though. Without any quantification it, to me it implies immediately (or as as immediately as reasonably possible).

I saw the picture in the question on Facebook. I can't find anymore the page (not FR24), but the caption was that the pilots weren't allowed to land and terminate the flight rather to find a suitable alternate outside of Europe. I don't understand why, but I don't have the authority to comment.

@Notts90 Some B737 Max were admitted to land in EASA area after the directive. See https://airlinerwatch.com/three-turkish-boeing-737-max-8-returns-to-istanbul/. Meanwhile, the Indians were a bit smarter. They indicated a date/time for the ban to become effective. https://www.thehindu.com/news/national/all-boeing-737-max-8-aircraft-to-be-grounded-by-4-pm-dgca/article26518572.ece?homepage=true Common sense is not evenly distributed, it seems.

@bogl The flights that Airliner Watch mentions being allowed to land in the UK had come from Alicante and Marrakesh. If they were close to their destination, they probably wouldn't have had enough fuel to land somewhere outside EASA jurisdiction. And, while the Marrakesh flight could, fuel-permitting, have gone back to Morocco, it's not clear where you'd even send a Spain-UK flight -- "Oh, sorry, you're going to Morocco, too!"?

@DavidRicherby, Not sure what is your point. How is that different from DXB-PRG?

@bogl The DXB-PRG flight wasn't yet in European airspace. The two flights that were allowed to land were both already deep in European airspace and might not have had enough fuel to leave. One of them was a flight entirely within European airspace. There was probably no alternative but to allow those flights to land in Europe.

The ban to land was for flights in airspace outside of Europe after the directive had been published, obviously.

@bogl you're talking EU bureaucrats here, not logical beings... They closed EASA airspace even for flights already in EASA airspace. E.g. several TUI flights were forced to divert to airports in southern Europe, inside EASA airspace, when caught in the closure while already in EASA airspace on the way to airports further north from departures in Africa and the Middle East.

@jwenting, I am a bureaucrat myself. IMHO, the authors didn't even think of airborne flights. This reminds me to the exitus of East Germany: https://www.youtube.com/watch?v=b8GzptqhT68

authors: Vladimir F Героям слава


text:
Why does the speed tape on the Lockheed U-2 go the opposite way?
I found this clip online that shows the speed tape on the Lockheed U-2 going what I perceive is the opposite direction to the 'normal' convention. On most aircraft, the higher values are above and so the tape scrolls downwards, but in this case it is mirrored.
Observe indicator on left for the first few seconds in the following clip:

Here's a photo as well from from Wikimedia Commons:

Why do this? On every aircraft I can think of it is the opposite.
Is it the fact that you pitch down, speed increases? Then you got the speed/altitude indicators moving in different directions, so I don't see how that would make it any more clear, especially when it appears it's the only aircraft doing this.

Dunno about that... but I found it amusing that a plane the like the U2 appears to have a piece of yarn on the nose for coordinating turns.

I would say it's intuitive because the tape moves upward if the speed increases. But then the ALT tape breaks that rule, so that can't be it.

Just noticed the backup EADI's speedtape follows the same rule.

In essence, you are correct.
It was a conscious decision to do it that way:
- when the nose pitches up, you're "pitching" toward the slower airspeed on the tape...
- and you're pitching toward the higher altitude on the altitude tape.
My question:  why do most aircraft have the higher speed near the top of the tape?  Is it "just because the altitude tape does it that way"?  Or is there a better reason.
Having flown the U-2 with the glass cockpit for over 10 years, it is intuitive and is "normal" for me to see that.

Hello Jon, welcome to Aviation.SE!   Follow up questions should be asked as a separate question, preferably with a reference to the original question, so they can attract their own answers.

This answer is a bit of a guess.
I think this design choice helps the pilot in manually flying in the coffin corner. By ensuring that the direction of movement of the horizon, altitude and speed tape all require the same stick input, less mental workload is needed to fly the aircraft.
If the nose of the aircraft is lowered:

The artificial horizon will move up
The altitude will decrease, hence the altitude tape moves up
The speed will increase, hence the speed tape moves up

Any visual clue from the display that involves an upward optical flow can be corrected by pulling the stick. This consistent behaviour reduces the mental workload.


text:
Can a controller clear you through an SFRA?
A pilot I know (lets call him Joe) recently had a minor run in with the FAA after he skirted the edge of the Washington DC Special Flight Rules Area (SFRA) while returning to Leesburg (JYO).  Here are the details:

Joe was returning from KOMH.
Joe was trying to get on the ground quickly to avoid a wall of convective sigmet from the west, moving east quickly and threatening to engulf Leesburg and his landing in a small GA plane.
Joe was in contact with the Leesburg Tower Note: Leesburg is NOT usually a towered airport.  However Leesburg has been a test site for mobile air traffic control tower and that tower was operational when this occurred.
Joe was instructed by the tower to "Proceed directly to Leesburg".  He immediately turned directly towards Leesburg (right turn shown on chart), received permission to land and landed.
Joe was provided with a number to call by the tower.
Joe was interviewed by someone in the FAA, told he had skirted the SFRA, received a warning and the case was closed with no further action.

Though the case was closed it was still the FAA's position that a violation of the SFRA rules had occurred.
What I find confusing about this case is that, if ATC instructed Joe to "proceed directly to Leesburg", doesn't that mean they cleared him through the portion of the SFRA he will need to fly through to comply with their instruction?
Research done for this question

I have taken the SFRA training required to fly VFR in the SFRA and have again reviewed this material after hearing of Joe's incident.  I've found nothing in that material that conclusively answers this question.
I have reviewed this question/answers about flying in the Washington DC SFRA.
I have reviewed this general question about flying in SFRAs.
I have reviewed the NOTAM associated with Ingress/Egress to/from Leesburg airport.
I have reviewed already asked questions and did not find this specific question having already been asked.

Did Joe file a flight plan? I controller can "clear" you to fly through it, but you will still be in violation of the rules if you did not file a flight plan and operate on an assigned transponder code. The controller may have opened one for him in flight.

@mins Leesburg is inside the DC SFRA, something that "Joe" should have known since it was his destination. That means he would have had to file a flight plan prior to departure.

@RonBeyer that is not correct.  Ingress/Egress in and out of Leesburg does not require a flight plan.  Joe would only have to (and did) squawk 1226.  This is detailed in the NOTAM I referenced in my post.

@bclarkreston It is, if you look at the charts it is about 1.5 miles inside the DC SFRA. The JYO website confirms this. This is something that "Joe" should have known prior to leaving his destination. Any flight within the DC SFRA requires a flight plan.

@RonBeyer you are correct, JYO is inside the DC SFRA.  You do not, however, require a flight plan to land their from outside the SFRA.  Again, I reference the NOTAM regarding this.  This also has it's own chapter in the DC SFRA Course, a required course for all pilots who want to fly VFR within 60 NM of the KDCA DME.  I've taken this course twice.  I should also note that I fly in and out of KJYO roughly twice a month.

An AOPA article on this has a chart of the Leesburg Maneuvering Area (LMA) which, as you said, doesn't require a DC SFRA flight plan (you do need one for pattern work, though):

If you compare the area in the chart to the flight path in the image in your question, it looks to me like Joe entered the SFRA southwest of JYO and then entered the south edge of the LMA. I suspect the FAA is saying that he should have flown further north, then entered the LMA from the west. In other words, he cut through a corner of the SFRA. I've marked that in red here:

The LMA NOTAM says:

PILOTS MUST ENTER THE LMA VIA THE MOST DIRECT ROUTE AND AVOID ENTERING
  REST OF THE DC SFRA

By cutting the corner, Joe entered a part of the SFRA that was outside the LMA, hence the call with the FAA. That's my interpretation, at least, although I'm surprised that the FAA didn't spell out to him very clearly what the issue was.
And to answer the question in the title, ATC cannot clear a pilot to do something he isn't authorized to do. ATC could clear a student pilot into class B airspace, but if the student doesn't have a logbook endorsement for that airspace then he still isn't allowed to enter (barring emergencies). "Proceed direct Leesburg" doesn't mean "fly in a straight line to Leesburg regardless of other considerations", the pilot still has to get there legally.

The last sentence sums it up nicely.  +1

@Pondlife, I've accepted (and voted for) your answer, but I'm still a bit confused.  FAR 91.123(b) says:
Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.
Wouldn't Joe have been violating this FAR had he not flown directly to Leesburg?  I know that the title of my question refers to a clearance, but now that I've thought about it a bit, I would actually call "Proceed directly to Leesburg" more of an instruction than a clearance.  Thoughts?

@bclarkreston This question is about exactly that issue. I'd sum it up like this: ATC cannot possibly know everything about what you and your aircraft are technically and legally capable of, and saying "unable" or requesting a specific flight path is not a violation of 91.123. Or, as one instructor said to me: "they don't like to hear it, but they work for you, not the other way around" :-)

@Pondlife, isn't there a difference between VFR and IFR in this case though? In IFR the clearance does have higher priority than restricted spaces, no? (in general; of course this is a small airfield where you wouldn't be flying IFR).

@JanHudec Well, maybe :) ATC can indeed vector aircraft through restricted airspace if they have procedures in place for doing that (ATC orders 9-3-4). As a pilot, though, I always ask before entering the airspace, just in case; ATC makes mistakes too. And I haven't done the SFRA training so it's possible that there are special rules that I don't know about there. By the way, VFR vs IFR doesn't really make a difference. If you're in controlled airspace you're required to follow ATC instructions and clearances regardless of flight rules (91.123).

Having a controller issue a clearance does not exempt a pilot from the regulations, nor does it establish a waiver.  FAA case histories are clear on that.  Furthermore, the rules for the SFRA are clear, and well publicized.
Remember that a pilot may refuse a clearance, and should consider doing so when they are concerned that accepting that clearance might result in a violation.


text:
Could an F-35 really be 3% rare-earth?
The Financial Times' China targets rare earth export curbs to hobble US defence industry states:

Fighter jets such as the F-35, a Lockheed Martin aircraft, rely heavily on rare earths for critical components such as electrical power systems and magnets. A Congressional Research Service report said that each F-35 required 417kg of rare-earth materials.

Rare Earth elements are commonplace rather than rare in high tech devices and systems these days, but usually in small quantities. The exception are rare-earth permanent magnets used in electric motors and generators.
But 417 kg sounds like a lot; it's 3.1% of a reported 13,290 kg dry weight of an F-35A.
Could this be true? Or are they counting for just for example the whole motor rather than the rare earth within it?

That seems unlikely, more likely there are 417kg of materials that have some rare earths in them rather than having 417kg of rare earths.

Here's the report they seem to be referring to;  see page 4.  (920 lbs ≈ 417 kg.)  The claim seems to stem from a DOD report to congress from September 2012 titled "Report on Feasibility and Desirability of Recycling, Recovery, and Reprocessing Rare Earth Elements" (referred to on p. 6 of this report), but I have been unable to find the report itself.  It wouldn't surprise me if the details are classified.

I would expect that to include all the rare earth materials in all the supporting equipment on the ground, plus everything that gets lost in the supply chain.

Agree 417 kg sounds like a lot.  As you say, the only bulk application of rare earths is magnets.  Why would an aircraft have half a ton of magnets?  The Congressional Research Service reports refers to 'rare earth materials' and says 'each F-35 Lightning II aircraft would require approximately 920 pounds of these materials'.  This references some other report which is apparently not public.  'rare earth materials' is pretty vague.  Guess:  the original report is referring to the quantity of rare earth ores required in the supply chain, rather than to end-product metals.

@ymb1 that's a lot of lanthanum! Okay I guess the answer is "ya, probably..." :-)

@ymb1 excellent, what a comparison!


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Is it always advisable for passengers to wear seat belts when a 747 is on final approach?
I noticed in this recent video that some of the passengers are not wearing their seat belts. I realize this is not a typical commercial flight, but is it advisable to wear seat belts when a 747 or similar aircraft is flying so close to the ground, (apparently) during a final approach? Or are the safety precautions here so thorough that it's not actually necessary in this case?

Related 14 CFR 91.107 but its important to note that the airplane in the pictures is Air Force One.

@RonBeyer thanks! I'm guessing then that this is not necessarily considered a "U.S.-registered civil aircraft," although if it were, the phrases like "...or gondola or an airship type certificated before November 2, 1987" or "Unless otherwise authorized by the Administrator—" might also come in to play. But in this question I'm really asking about safety and advisability, not regulations.

AFO is owned and operated by US Air Force and is subject to different regulations. Its always advisable to wear a seatbelt on an aircraft as an accident or malfunction can occur at any time.

For every person who advises to do something, you'll find another who advises not to. As a pilot though I would recommend wearing it as much as possible, there is always unexpected turbulence or other things that can injure you or the people around you.

You've asked a question the answer to which is a matter of opinion. My opinion is that when you're in a seat, whether a pilot seat or a passenger seat, it's best to have the seat belt fastened, though you may have the belt adjusted so it's comfortably loose. However, in reasonable flight conditions I have no problem with unfastening it to move about the cockpit or cabin. When sleeping across seats, I usually did not use a belt. In 3-man cockpits of 727s and 747s, I used to sit crossways in the captain's seat for extended periods while talking with the f.o. and f.e. without wearing a seat belt.

@Terry So was that captain's prerogative or is that one off the things you can fess up to now being retired? :)

My personal rule is always to wear my seat belt unless I want to get up. Having said that, I once landed standing up, holding on the back of a C-130 pilot's seat. If everything is going right, seat belts are pretty unnecessary. It's just when things are going wrong that you wish you were strapped in.

Consider that this may be an "all eyes" situation.  Since this is a new landing zone, could it be that "all eyes" have been requested to scan for threats?  I know this is a stretch, given the level of security involved for POTUS, but I know it is done in other military applications.

I was going to say something funny @Keeta, but actually, considering this particular situation overall, maybe!

Actually @HowardMiller it's about 10 seconds before something goes wrong that I would wish that all of the other passengers around me were snapped in too!

@Keeta 'this may be an "all eyes" situation". Very unlikely that it's important enough for the President to be watching for danger, but not the photographer taking the picture. Almost certainly a case where the publicity value of getting a picture of the President arriving in Havana outweighs the tiny risk of him not wearing a seatbelt.

@TomMcW There was nothing in our ops specs or for that matter in FAA rules back then (1990s), at least as I remember, that required seat belts or shoulder harnesses during cruise flight. I get the impression, but do not know, that today's cockpits are much less relaxed than we were back then.

@HowardMiller I made trips standing up in 727s from Terre Haute through Minneapolis to Seattle and vice-versa in the late 1980s. It was a night freight operation with all the pilots based in Terre Haute, but a lot of us lived in Oregon and Washington. Two jump seats filled, one sitting on the toilet, two standing was normal.

@Terry Standing up!? Like a flying jitney! My company would've had none of that in the 90's. They made us shave our beards to jumpseat because of the 02 masks. But I did get to do what you were saying and fold up the armrests and sleep across the whole row. The 47's were nice to jetmpseat in. Too bad we only had two of them.

@TomMcW I doubt the company ever knew what we were doing. Insofar as sleeping on a 747, we used to ferry 747s empty except for half a dozen cockpit crews, a purser, and a few maintenance people to and from Jakarta for the Hajj. What those not operating would do was to put up all the arm rests for a middle row (5 seats), then take the seat cushions from the row in front and the row in back to further pad your sleeping row. Made for a very comfortable bed.

@Terry Was there, at that time, no 91.105 requirement that crewmembers "keep the safety belt fastened while at the crewmember station"?

@JonathanWalters Your comment made me curious, and I found http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/910628D28B527432852566CF00613CBE?OpenDocument which seems to indicate that reg was first published in Oct 1992. My 727 and 747 flying started in 1988 and extended through 1992. Thus it would appear I started my jet experience without that regulation. Frankly, I don't remember it being publicized, but it's been a long time since now.

It is, of course, advisable to wear your seat belt, yes. However, there appears to be somewhat of a tradition of not always wearing seat belts on Air Force One:
Stanley B. Greenberg, Dispatches from the War Room: In the Trenches with Five Extraordinary Leaders (Google Books):

Nobody wears seat belts and everyone uses their cell phones while the
  plane is taking off.

Presumably the crew would provide a special warning if they expected a rough ride.

There are multiple good answers here, but you've also taken the time to find additional supporting data. So in addition to the peanuts, M&Ms, pens, etc. there are other perks that come with this job - you get to not wear your seat belt sometimes. Thanks for the heads-up!

Yes, it is always advisable for passengers to wear the provided safety belts in all phases of flight. This is especially true for aircraft on takeoff, landing, in low level flight, or in any other flight regimes that are especially prone to turbulence or other risk exposure.
A large, heavy aircraft—such as a B747—will be less affected by wake or mechanical turbulence than would a smaller aircraft—such as a CRJ200 or a C172. However, even large, heavy aircraft can be subjected to severe turbulence that could result in injury or death if passengers are not restrained by appropriate safety belts.
Additionally, for flights conducted under 14 CFR 121—which I am confident is a majority of the US operated passenger flights involving the B747 (though not, obviously, Air Force One pictured in the question above)—passengers are generally required to:

occupy an approved seat or berth with a separate safety belt properly secured about him or her during movement on the surface, takeoff, and landing
Excerpted from 14 CFR 121.311 (b)

It might also be worth noting that, at least in the U.S., the same requirement applies to GA, per 14 CFR 91.107. So, even if it's a privately-owned 747, the seat belt requirement is still in place during taxi, takeoff, and landing.

Yes, this is also noted in the comments under the question.

This is also a correct answer - and is up-voted more because the message is important. Always keep your belt fastened - even when the light is out - even if you think you are in control of the situation. Stuff happens.

Don't worry about what Obama does in particular . It's always advised to wear seat belt whether you are in Boeing 747 or other small planes.
According to this Telegraph.co.uk article:

"Seat belts are required and designed to ensure passenger safety and reduce the risk of injury, particularly during deceleration, turbulence and unplanned or difficult to predict events.”

And later on in the same article:

Last year, severe turbulence caused three EasyJet crew members to “hit
  the ceiling” over Italy, according to a passenger eye-witness, with
  one sustaining a hip injury.

Now all that can be said is if you don't want to be next victim wear the seatbelts.

Wow thanks now it sounds & looks better.


text:
Can one tell if a craft is a lifting body or a flying wing just by looking at it?
Recent news that the "reusable lifting-body spaceplane" Dream Chaser has arrived at Kennedy Space Center and may launch in August launched led me to read Wikipedia's Lifting body which begins:

A lifting body is a fixed-wing aircraft or spacecraft configuration in which the body itself produces lift. In contrast to a flying wing, which is a wing with minimal or no conventional fuselage, a lifting body can be thought of as a fuselage with little or no conventional wing. Whereas a flying wing seeks to maximize cruise efficiency at subsonic speeds by eliminating non-lifting surfaces, lifting bodies generally minimize the drag and structure of a wing for subsonic, supersonic and hypersonic flight, or spacecraft re-entry. All of these flight regimes pose challenges for proper flight safety.

Trying to distill that, I'm seeing a minimization of surfaces that don't provide lift in both. To me, one is "almost all wing" and the other "almost no wing" where the definition of "wing" in this context is left to the readers imagination.
Suppose a large number of 3D models of craft capable of flight and with relatively low drag at low speed but having a wide variety of supersonic drags were presented to a group of aerospace engineers "skilled in the art".
Would they be able to classify them as either "flying wing-llike", "lifting body-like" and "winged airplane-like" based on visual appearances with significant agreement? If so, is it possible to explain objectively how to tell the difference?
Or is it more of a Duck test or I know it when I see it?

I think it's pretty easy: A washing machine is a lifting body, a washing machine with wing is an airplane, a washing machine with a rotor is a helicopter, a washing machine with at least four rotors is a drone/UAV, a washing machine shaped like a wing is a flying wing

@sophit Hmm... how does one treating "shaped is like a wing" as  boolean? By what criteria does an observer arrive at a True/False value? Personally, I think that it is the opposite of "pretty easy", but if you can post a convincing answer that is well-received, I might be able to see the ease more clearly.

I was just joking  Anyway if the payload is in the wing, i.e. there's no ad-hoc volume (fuselage) for it, then it should be a flying wing. Maybe there are exceptions but it should be a pretty solid definition (?)

@sophit can you post it as an answer, and see if it "flies"?

I think you should not check too strictly the definitions. It is mostly an engineering way to think (and to optimize a design). Wings have problem at the blade (at supersonic speed), large bodies not (air will create a cushion), but without wings you lose most controls (leverage, farther from center of mass) so you need engines or reaction trusts. And in any case every design is affected by many trade-off, so forget definition, it is mostly about pedigree of original design/revolution/evolution), and people use it to seems smarter.

"how does one treating "shaped is like a wing" as boolean? By what criteria does an observer arrive at a True/False value?" Step 1:  Let go of trying to do this.

Actually, they are both flying wings, or, they are both lifting bodies
A "wing" is simply something with a higher coefficient of lift.
Coming back from orbit, at around 17,500 mph, the V$^2$ part of the lift equation takes care of producing lift even though area, angle of attack, density, and coefficient of lift may be smaller.
Even the Space Shuttle, considered "winged", had a landing speed of over 200 mph.
A "lifting body" trades coefficient of lift for strength.
High aspect ratio wings remain most efficient at producing lift, but are not as strong.

I'm trying to see how this fits "... just by looking at it?" (the question's title) The body of the question reiterates "Would they be able to classify them... based on visual appearances with significant agreement?" Right now this nicely answers a question I didn't ask.

@uhoh in reality application determines design, so that may provide a more clear cut understanding.  "Just by looking at it", they both produce lift.  Interestingly, a low aspect, stumpy design is much better at handling re-entry, the most dangerous part of the flight.  Once subsonic, the wing wins efficiency hands down, but orbiters need only a longer runway and good tires.

If the answer to the question is "No, not really, because..." then that would be helpful to add.

I think that the historical perspective is the best way to tell those two classes of aircraft apart.

Flying wing
Between the world wars, both the Horten brothers of Germany and Jack Northrop of the USA experimented with the idea of striping out all the "useless" part of a conventional airplane in order to get the "purest" form of flying machine. Their idea was that tailplanes, fuselages, nacelles,... everything but the wing adds weight, drag and complexity and should therefore be eliminated and its function gathered in the wing itself. Their idea was actually not wrong and both the Ho 229 and the YB-49 proved to have exceptional flying characteristics for the time. This is an example:

No fuselage, no tailplane, no nacelles but it flies! (Gif source)

Lifting body
"Aerospace-related lifting body research arose from the idea of spacecraft re-entering the Earth's atmosphere and landing much like a regular airplane. Following atmospheric re-entry, the capsule spacecraft from the Mercury, Gemini, and Apollo series had very little control over where they landed. A steerable spacecraft with wings could significantly extend its landing envelope. However, the vehicle's wings would have to be designed to withstand the dynamic and thermal stresses of both re-entry and hypersonic flight. One proposal eliminated wings altogether: design the fuselage body to produce lift by itself." (from Wikipedia). This is an example:

A lifting body doesn't fly, falls with style. This Northrop HL-10 needed a B-52 to go in the sky (source Wikipedia)
And this is how they landed sometimes (no worries, the pilot survived and returned to fly!):

That being said and not pretending to be general or definitive, we can say that:

If the main purpose of the aircraft is to fly, possibly more efficiently than a conventional subsonic airplane, then we have a flying wing.
If the main purpose of the aircraft is simply "falling with style", possibly from supersonic speeds, then we have a lifting body.

Therefore, a flying wing can be recognised by the fact that:

it can take off, fly and land on its own feet;
it doesn't fly supersonic;
the fuselage is "laterally stretched" in the shape of a wing in order to be used not only to transport the payload but to generate the needed lift as well, with the goal of being more efficient than a conventional airplane;
it lacks proper nacelles and vertical and horizontal stabilisers;
it's powered by conventional aeronautical engines.

A  lifting body can be recognized by the fact that:

it cannot move from the landing field unless another machine brings it in flight;
the fuselage doesn't resemble a wing i.e. it doesn't create much more lift than drag;
it falls with AoA bigger than some 10/15° (stall angle of a conventional wing);
if something resembling a fin/airfoil is present, then it's normally mounted almost vertically and mainly to provide stability;
it might have some form of heat shielding;
in the mission profile there might be a phase where it actually flies powered by a not conventional aeronautical engine, like a rocket.

Now I let you decide what this thing is (picture from Wikipedia)

OTOH the photos in the linked wikipedia article show something more like an aircraft then a spacecraft,

"... a wing is present then it's normally mounted almost vertically, ..."  errr, what??

@sophit  Read again:
"A lifting body can be recognized by the fact that:
it cannot move from the landing field unless another machine brings it in flight;
the fuselage doesn't resemble a wing i.e. it doesn't create much more lift than drag;
it falls with AoA bigger than some 10/15° (stall angle of a conventional wing);
if something resembling a wing is present, then it's normally mounted almost vertically and mainly to provide stability;"
Are you saying that a vertical stabiliser "looks like a wing"?

@MikeB what about "lifting surface"?

I don't like "lifting surface".  Because everyone understands that lift opposes gravity, so no vertically mounted fin would ever be mistaken for a lifting surface.  I'd go back to "something that looks like a wing", or fin, or airfoil, or protuberance, or...

@MichaelHall most people here at least. Since space planes and reentry vehicles have been brought up here, I'll mention that for those folks, drag is aerodynamic force along the thrust axis and lift is any aerodynamic force perpendicular to it. Thus during reentry a washing machine can indeed be a lifting body as long as it's tilted.

@MichaelHall for more on that stuff, there's these in Space SE: Rocket drag and lift based on flight direction - in which frame of reference? and Is aerodynamic lift ever useful in rocket flight? and for fun What would a "Kármán plane" look like, a bird, or a plane?

@uhoh, I get what you are saying, and normally try to avoid absolute statements, but literally every person on the planet feels gravity and has a concept of what is needed to oppose it.  (i.e. "lift" the rock up off the ground...)  No special aviation knowledge is needed, and even a 10 year old paper airplane flyer understands that a vertical airfoil won't provide any lift.

@user721108, from Wikipedia:  "A lifting body is a fixed-wing aircraft or spacecraft configuration in which the body itself produces lift."  A washing machine does not meet this definition.

@MichaelHall, well at 22.5° of AoA a square has a $C_l$ of 0.15 (and an outrageous $C_d$ of 2)... So, a washing machine does indeed meet the definition

@sophit, not by the definition of the term I posted above.  Now... if you can find a credible reference that says "A lifting body is an appliance for washing clothing in which the body itself produces lift" I might be willing to concede.  ;)

Since the question is specifically about visual identification it probably helps to get some pictures up on the screen.
Flying Wings

Artist impression of the Horten H.VII (author FOX 52, via Wikimedia Commons,  CC-BY-SA-4.0)

Horten Brothers flying wings (author Greg Goebel, via Flickr, CC BY-SA 2.0)

YB-35, YB-49, B-2 (author Nacht Eule, via Wikimedia Commons,  CC-BY-SA-4.0)

Wikipedia defines a flying wing as:

a tailless fixed-wing aircraft that has no definite fuselage, with its crew, payload, fuel, and equipment housed inside the main wing structure.

A Universe Today article adds some additional visual specifics:

From the top, a flying wing looks like a chevron (inverted V), with the wings constituting its outer edges and the front middle serving as the cockpit or pilot’s seat.

Although not necessarily authoritative, it's an interesting point "the wings constituting its outer edges" as that is definitely a visual indicator, and seems to imply that top-down is a primary angle for visual identification. For example the B-2 flying wing when viewed from the side seems to have something of a discernible fuselage, however this does not change the shape of the "outer edges" as seen in a top-down view.

B-2 Spirit
Lifting Bodies
As defined by Wikipedia:

In contrast to a flying wing, which is a wing with minimal or no conventional fuselage, a lifting body can be thought of as a fuselage with little or no conventional wing.

Visually speaking, lifting bodies seem to be distinguishable from most angles by the absence of a wing:

X-24A, M2-F3, HL-10 (NASA)

(NASA)
However the top-down view may not always be sufficient, for example the X-20 Dyna-Soar has a similar top down chevron shape as the B-2:

From "Frontiers of Space" by Philip Bono & Kenneth Gatland, 1969 (via Flickr, CC BY-NC 2.0)
The distinguishing feature in this case seeming to be the "winglets" which act as a tail:

X-20 Dyna-Soar (NASA)
But the X-20 Dyna-Soar spaceplane is also not really a lifting body and is more of a blended wing design. The the distinction between lifting bodies and blended wing aircraft is a separate discussion outside the scope of this question. Some examples of blended wings are more obvious such as the NASA X-48 prototype:

X-48 (photo by Alan Radecki, via Wikimedia Commons, CC BY 2.5)
or the X-47B:

X-47B (U.S. Navy)
Although not a lifting body, the X-47B seems similar in shape to the B-2 flying wing, and it is also tailless. However circling back to the Universe Today definition it is not quite as angular of a chevron shape as the B-2.
Spaceplanes such as the Space Shuttle and the X-37B are often described as lifting bodies:

(NASA)

X-37B (NASA)
Although while they have lifting body characteristics, the wing size would seem to make them more of a blended wing design than the lifting body spaceplanes such as the Dream Chaser or the X-33 concept design:

Dream Chaser (NASA)

X-33 (NASA)
Again lifting body vs. blended wing is a separate discussion which does not seem to affect the visual distinction between flying wings and lifting bodies.
Of course aeronautical engineers would be evaluating these type of images based on their knowledge of the functional differences between flying wings and lifting bodies, the type of details mentioned in this answer. How engineers would translate those technical definitions into visual identifiers which can be described in terms of shapes and relative component sizes, would probably be the definitive answer to your question.

I declare you the winner!  (@sophit, i wonder why Steve didn't include any washing machine photos?)

MichaelHall - copyright free photos of flying washing machines are hard to come by.

@MichaelHall: sorry, I didn't know you folks needed a picture, next time I'll do even better and add a onlyfans link as well


text:
Why is the 5G rollout a problem in the USA but apparently nowhere else?
The FAA is concerned that 5G cell phone signals could adversely affect radar altimeters on aircraft.  This is because 5G signals are in the 3.7 to 3.98 GHz range, while radar altimeters operate in the 4.2-4.4 GHz frequency range. The worry is that the nearby cell signals will cause interference with avionics.  However the 5G roll-out in Europe is also under way but I haven't noticed any concerns raised by EASA or the CAA.  Why not?  Is there a difference in frequencies/equipment between the US and Europe?  Or have I just missed the news stories about potential problems outside the USA?

"nearby cell signals" - There's 200 MHz of separation!  If this sort of thing was going to be an issue, wouldn't we have long since seen issues with VORs from the FM broadcast stations in an immediately-adjacent band?

Since you are asking specifically about the 3.7–3.98 GHz band, the answer is that most of the world uses the 3.3–3.8 GHz band. The 3.7–3.98 GHz band was seen as much less important until US operators (somewhat surprisingly) paid over $90 billion in an auction this year.
So, the US are the first (and at the moment only) ones to use this band, which is why it makes sense that they are also the first ones to worry about interference in this band.
However, the French DGAC has also voiced concerns.

"most of the world uses the 3.3–3.8 GHz band" is inaccurate. The EU utilizes 3.4-3.8. Canada allocated 3.475-4, China is 3.3-3.6, but critically Japan has allocated 3.6-4.1, and South Korea allocated generally 3.4-4.

So how is the lower 3GHz used by the US right now? US is the first region in the world to use mm wave (~30GHz) as well. Unlike other regions, US seems really short on sub-6G, e.g. lower 3GHz and 5GHz.

@user3528438 AFAICT, the highest band currently used in the US for mobile networks is B41 (2.5GHz).

StephenS has provided a good answer however I'm going to accept this because Jörg specifically addresses the difference between the US and the rest of the world.  I guess my problem now is to find which stack I should use to ask if the handsets differ for different frequencies.

@DaveGremlin Let me know where you ask and I’ll answer. Short version: handsets sold in different regions support different sets of bands, but today they should all include a small subset that works everywhere as a last resort.

If you read the FAA’s statements (not media reports) closely, you will find their complaint is merely that it hasn’t been proven not to be a problem, not that they actually expect a problem or have any rational basis to suspect it would be.
This follows the general hyper-conservative stance of the FAA: assume everything is unsafe until proven safe (at someone else’s expense), which has not been done in this case.
As a result, operators are prohibited from relying on radioaltimeters in areas where the 3.7-3.98GHz band is being rolled out, but they are still to monitor it. If no problems are reported after some period, they will rescind the order.

+1  My guess was that there would be a standard 5G frequency and the FAA would look at evidence from other nation's 5G rollouts when making a decision, but regulation is never that simple in aviation.

@DaveGremlin Every country is free to allocate its radio spectrum however it wants, and the result is a mess of different mobile bands. In the olden days, you even had to rent a local phone when you traveled because your own might not support the bands of the country you were visiting.

I'm pretty sure that the FAA assumed the 737 Max was safe until repeatedly proven unsafe. Hyper-conservative is not the word I would choose. I might consider this more of an over-reaction in the wake of the 737 Max affair. As Vikki already noticed, it's 200 MHz separation. 5G specs allow a 100PPB+15Hz frequency error, i.e. 415 Hz at 4 Ghz. That's 500.000 times less than the 200.000.000 Hz separation.

@MSalters - I might be wrong, but I think that's just for the carrier frequency.  As soon as you modulate the carrier (in order to carry information), the bandwidth increases.

And filters (used in receivers, such as the receive portion of an altimeter), are not perfect.  That means that an altimeter using 4.2 GHz MAY see some leakage or bleed over from a 3.98 GHz cell phone signal.

@MSalters Boeing provided the FAA with tons of (falsified) data that proved the MAX was safe. That’s different from 5G on the 3.8GHz band, where there is currently no data at all.

"not that they actually expect a problem or have any rational basis to suspect it would be."  False.  RTCA Paper No. 274-20/PMC-2073 reports modeled interference with radio altimeters arising from intentional 3.7-3.98 GHz and spurious 4.2-4.4 GHz emissions by 5G radios.  Worst-case interference is more than 40 dB over the measured safe interference limit for those altimeters.  This report was published October 2020 (before the FCC's C-band auction).

OT: #TIL that the aviation industry's charming term for a plane crash is "CFIT", or "Controlled Flight into Terrain".

@FeRD Not all crashes are CFIT.

@MSalters 200MHz corresponds to 5 ns in the time domain, or 2.5 feet uncertainty in a radar signal round trip. So the receiver may be fairly wideband to keep good time domain accuracy. I'm not sure you want more altimeter error than that at touchdown. Anyone have info on actual radar altimeter accuracy?

and sources e.g. https://www.aviationpros.com/home/article/10387134/radio-altitude-the-instrument-of-choice put radar altimeter accuracy close to ground at 2 feet, or (other sources) 0.75m (2.5 feet). So there may well be time domain effects requiring high bandwidth.

The FAA wants to draw attention to and monitor any effect of 5G on aviation safety so it detects issues now, while they are minor, and not later, as the number of 5G devices per square mile grows and the effects get stronger.
The rest of this is just my theory on why it might be different by nation.  In the USA, there has always been an abundance of extreme caution about allowing radio equipment to proliferate in the market before ruling out the potential that it could later have to be banned or heavily restricted.  You want to know up front.  This eliminates waste in producing such equipment.  The national difference here is the "regulatory takings" doctrine, which I don't think much of Europe has.  That is where, if a person or company bought a perfectly legal thing that had a perfectly legal use, and that use is later banned or restricted in a way that decimates its value, it's considered "taken" by the government, even if they didn't physically take hold of it.  This means it must be for the public good, and even then, they have to pay.  If you ban beef, you've "taken" the full value of all the cows currently being raised for beef, and would have to compensate farmers their full value under the Constitution.  Ban driving, pay for all the cars.  There are still exceptions to seat belt laws for extremely old cars that existed before the seat belt law - Congress keeps those exceptions because if they didn't have that exception, the law would be a "taking" of antique cars and they'd have to pay for them.  Ban 5G later when problems are discovered, pay for all the towers (and all the phones' price difference from the 4G models most likely).  Because the financial risk of proliferating infrastructure that will have to be banned falls on the agency banning it, they have an incentive to actually look ahead before more is built.  If you are a European leader, ignore the problem until it is a crisis, then ban 5G, and make the telco's take the loss.


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What are the financial implications for Lufthansa (and other airlines) of keeping the back row of their A320neo free of passengers?
After the recent EASA airworthiness directive concerning the potential for weight and balance problems with the A320neo, Lufthansa announced it would no longer sell seats on the back row of those aircraft.  What are the financial implications for airlines of not being able to fill an aircraft?

I'm not sure what you mean by "implications"? They don't sell them, so they don't get the # of seats * expected revenue for each flight. Can you specify exactly what you are looking for?

@RonBeyer I guess the question boils down to how often were Lufthansa's A320neos full enough to actually need the last 6 seats on the aircraft.

@Ron Beyer - Financial implications = How much will airlines lose because they aren't going to sell those seats?

Not being able to sell seats on the last row would (in Lufthansa's case) theoretically mean a 3.3% loss of revenue: LH setup for A320neo is 180 seats, 6 seats in last row.
But, only theoretically, because seat occupancy rate for Lufthansa Group was  81.4% (2018). It is very unlikely that all of the flights operated with A320neo would fly 100% fully booked. In a non fully booked flight, the last row would obviously be the first one to be left empty. The actual loss of revenue would be way less than 3.3%, but how much, it is impossible to say.
Anecdotally I'd say that it is a blessing the last row is out of service for now, this Business Traveller article will give you an idea  why: no window, seats do not recline, legroom is smaller...
World Airline News: Lufthansa Group becomes the largest airline group in Europe with 142 million passengers in 2018

Airlines, especially the biggies, usually have performance guarantees and AD clauses in their sales contracts that usually force the OEM to eat the financial hit, so they will be getting compensated by Airbus in any case.

Keep in mind that self-loading cargo is only part of what an aircraft carries, removing half a dozen passengers worth of weight means that there is more weight available for other cargo.

Excellent answer, +1 and 'accepted'.  I am a bit surprised at those levels of occupancy since I've seen quite a few flights overbooked and been bumped off them myself a couple of times


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Do business jet beds have safety belts?
I regularly notice reports of injuries due to turbulence on AVHerald.  I've also seen photos of bedrooms in aircraft such as the Boeing BBJ.  Are beds fitted with 'seat' belts to stop you hitting the ceiling in the middle of a nap should the aircraft run into turbulence?

I would think they would be waken up if the pilots anticipated turbulence ahead

@mins  That sounds like the basis for an answer.  AC 25-17 looks to be quite comprehensive.

Since there are some relatively large planes that are officially limited to 19 passengers for various regulatory reasons, I suspect (but just a comment, I don't know for sure and hopefully someone more knowledgeable will answer) that this may be a reason to not provide seat belts at extra seats. For example, if you have 19 regular seats in one part of the cabin and a "conference room" with 19 seats in another section of the cabin then if you put seat belts in all the seats someone in the govt. could argue that this is really a 38-seat plane having additional staffing & safety requirements.

Overall
US regulation doesn't require every seat/berth to be provided with restraints.
It requires there are approved seats/berths with restraints in appropriate quantity and they are used during taxiing, takeoff and landing. Approved berths with restraints are required only for non ambulatory persons.
So if we except non ambulatory persons, a different topic, each occupant of a private aircraft is provided an approved seat with a restraint, but there can be additional seats and berths, showers, massage tables, rocking chairs without safety restraints.
The seat allocated must be used during critical phases of the flight, and the crew may ask passengers to return to their approved seat at any time, and fasten their approved seat belt.
This applies to private transport or parabolic flights like it applies to public transport. Whether regulation can be enforced by the crew is indeed a separate topic.
US legislation
Regarding number of seats vs. number of passengers
Aircraft requirements are described in 14 CFR § 25, Airworthiness Standards: Transport Category Airplanes. At 25.785 Seats, berths, safety belts, and harnesses:

(a) A seat (or berth for a nonambulant person) must be provided for each occupant who has reached his or her second birthday.

Each ambulatory passenger must have a seat, each non ambulatory passenger must have a berth.
Regarding when to use safety belts
Regulation is found in 14 CFR § 91, General Operating and Flight Rules, at 91.107 - Use of safety belts, shoulder harnesses, and child restraint systems.

(a) Unless otherwise authorized by the Administrator—
(3) Except as provided in this paragraph, $\sf \color {OrangeRed} {\text {each person}}$ on board a U.S.-registered civil aircraft [...] $\sf \color {OrangeRed} {\text {must occupy an approved seat or berth with a safety belt}}$ and, if installed, shoulder harness, properly secured about him or her $\sf \color {OrangeRed} {\text {during movement on the surface, takeoff, and landing}}$.

Each seat used by an ambulatory passenger or crew during the critical phases (taxi, T/O, landing) must be approved, and equipped with a belt or a harness.
Regarding medical aircraft
Non ambulatory patients are central to this type of aircraft, recommendations are provided in 23.785 Seats, berths, litters, safety belts, and shoulder harnesses. An example of a simple approved restraint can be seen here.


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Why does "airplane mode" turn off Bluetooth and WLAN if I can just turn them on again?
On my (Android) phone, enabling airplane mode automatically disables all wireless antennas except for GPS and NFC (I assume the latter is either passive or way too low power for anyone to care about).  In particular, it disables mobile data (4G/LTE, or whatever it has roamed onto), WLAN (WiFi), and Bluetooth.
However, I can turn WLAN and Bluetooth back on again without disabling airplane mode.  Asking the airline, as suggested in this answer, is useless.  During the safety briefing of every flight I remember in the recent past, they would say something along the lines of "You can use electronic devices as long as they're in airplane mode," or very rarely, "You need to switch your electronic devices completely off until we hit 10k feet, and then use them in airplane mode."  But they don't tell you anything more specific in the safety briefing or on the information card (e.g. "WLAN is fine, but keep Bluetooth off" or similar).  At the same time, most airlines clearly want you to turn on WLAN so that they can sell you internet access.
The obvious interpretation is that you can do whatever you want as long as your device is in airplane mode ("They said to put everything in airplane mode, and I did, so I complied with their instructions.").  But that actually doesn't make sense.  I own multiple devices which only have Bluetooth and/or WLAN antennas (e.g. the Nintendo Switch, most smart watches, and probably a truckload of other non-phone devices).  In most cases, you can turn both antennas back on after enabling airplane mode.  At that point, airplane mode is just a pretty little icon that appears in your device's status bar.  It doesn't actually do anything (but is presumably still required).
What is the point of disabling Bluetooth and WLAN if you're not prohibited from turning them on again?  What is the point of even having airplane mode on devices that don't support mobile data in the first place?

This question is no different than https://aviation.stackexchange.com/questions/8188/airplane-mode-does-not-disable-gps-is-it-safe-to-use-gps-in-flight which has been open for years.

"Airplane mode" is a convenience feature provided by the phone manufacturer. You can choose to turn it off anytime in violation of the rules, so there is no difference there.
What it does may not match the regulation in any particular geography as rules vary. Regardless of any particular device the airline sets rules to ensure that no device:
1) Interferes with navigation signals required by the aircraft, particularly at landing
2) Confuses cellphone towers on the ground which expect radio signals in roughly a planar orientation near the ground
The airline gets its rules from the FAA/EASA... and indirectly from the FCC... What was considered "airplane mode" by the FAA changed in 2013. Testing determined that wifi and bluetooth did not cause sufficient interference with ground based nav signals, so that prohibition was relaxed. An old phone's "airplane mode" may be too restrictive.
There was no change to the rule on cellphone voice communication, which remained prohibited. Once the aircraft is sufficiently high no ground based cell tower can receive any signal, so this prohibition mid-flight is effectively meaningless.

Additionally a lot of flights allow WiFi connections to watch entertainment or use bluetooth so you can listen to music with your headphones.

The main headache from active cell phones on board is a kind morse code deet-deet-deet-deet sound that comes over the VHF com when a cell phone is transmitting.

"Airplane mode" is a convenience feature Continuing on this, devices may just turn off certain features because it meets the customer's expectations. For example, turning off WiFi may save battery for flights that don't have onboard charging.


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Are the spring constant and damping coefficient of landing gear publicly available?
I'm trying to find the spring constant and damping coefficient for a Cessna 172. I looked through the POH, but couldn't find them anywhere. Are these values available publicly in some design document I'm not aware of? (I'm trying to determine the force needed for the plane to bounce.)


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767 fuel line issue
Recently flew a 767 that needed to turn back after about 45 minutes b/c, according to the pilot, fuel was only being pumped from one tank, which could lead to an imbalance if left to continue.
Generally in small GA aircraft one of the first things you do on the ground is switch fuel tanks to ensure proper flow from both. Is this also standard procedure on a 767? If so, what’s likely to have happened in-flight?

Just to clarify, you're asking if the pre-flight checks on a 767 include testing that the fuel transfer pumps are working?

Yes, and if so, what could cause them to not work within 45 minutes of take off

You said they squawked 7700, what was the airline and flight number? And how did you know they declared an emergency? There’s likely more information available.

TL;DR: Large aircraft normally isolate the left and right fuel systems, so if engines start, you have enough accessible fuel to take off and divert or return. There is also a cross-feed mechanism to rebalance fuel in flight, so most failures can be corrected.

Generally in small GA aircraft one of the first things you do on the ground is switch fuel tanks to ensure proper flow from both.

Small GA aircraft have one engine and two wing tanks, so there is a selector valve and potential latent faults, e.g. a blockage affecting one tank. A jet transport, like a 767 has at least two engines and at least two wing tanks. In normal configuration, each engine solely feeds from its own wing tank, so if the engines start and run, it is a sign that each side is functioning properly.
Aircraft also have a cross-feed system, which is not normally used, but is important during an engine failure. In particular, a stuck-open crossfeed valve may cause fuel to preferentially feed from one side, leading to an imbalance. This isn't an uncorrectable problem, as you can use the cross-feed to rebalance the fuel via switching on and off fuel pumps, but a return is likely warranted.
Where the isolation breaks down is the center tank(s). Here, you have one center tank and separate pumps to transfer the fuel to each engine. Potentially, one of the center tank pumps failed, so only one engine could be fed from the center tank, which would eventually cause an imbalance. This could be problematic if the cross-feed valves were to fail.
Airbus in particular inhibits center tank feeding on takeoff for isolation purposes, so you can take off and then realize your center tank fuel is unusable. Boeing does let you feed from the center tank at takeoff. In both cases, this isn't a safety issue, as you burn center tank fuel first, and you have more than enough wing tank fuel for a return or diversion.
Another issue is fuel leaks. The fuel system consists of a complex series of piping, motor-operated valves, and check valves. Some failure modes can cause fuel to flow in an incorrect direction. Fuel piping is, to the greatest extent possible, contained inside tanks, so a leak won't result in a hazardous spill, but may manifest as a unwanted fuel transfer.

With a failed center tank pump & an operative crossed valve, you can still feed both engines from the center tank without causing any imbalance.

@RalphJ Right, I see the procedure for that. I changed the text a bit.

If a center tank pump fails, you immediately open the crossfeed valve so you're not creating an imbalance. If that fails to open, you have 2 unrelated failures at once, and you'd turn off the other center tank pump (to maintain balance) anf you'd land. But, if the crossfeed opened, you could continue (ETOPS scenarios aside) since the crossfeed valve's failure mode is a failure to move, not an uncommanded closing.

Would any of this necessitate squawking 7700? My flight did, but based on your answer I wouldn't have expected the pilot to do so.

@cph2117 The issue would be the early return. An overweight landing requires an emergency declaration.

This, and several similar incidents like it, sounds like the Boeing 767 fuel imbalance anomaly, which sometimes occurs when there enough fuel is on board to require putting fuel in the center tank.
Boeing normal procedures for the fuel system require using center tank fuel first, so center tank pumps will be on from the point of engine start.
The problem manifests itself when one of the center tank pumps is not producing enough pressure to override the pumps in one (or both) of the wing tanks, resulting in a simultaneous decrease in center tank fuel level and in one (or both both) wing tanks.  When it's just one wing tank involved, that results in a fuel imbalance.
It is not known why this anomaly occurs, and it is not predictable, but it appears to be related to an Airworthiness Directive from 2001 which superseded a 1997 AD that was issued after the Swiss Air accident.
The developing fuel imbalance presents two potential problems:

There is a fuel imbalance limitation of 1500 to 2500 pounds in the 767, depending on wing fuel quantity.  Continued flight in the anomalous condition could result in exceeding that limitation, although I am told that as a practical matter, the 767 flies just fine with one wing full and one wing empty.

Continued flight in the anomalous condition could eventually result in completely emptying the affected wing tank, resulting in an engine flameout on the affected side.

My company's AOM (Aircraft Operating Manual) directs the crew to an appropriate supplemental normal or non-normal procedure when the anomaly occurs.  Presumably, other 767 operators have similar instructions in their manuals.
I have seen this anomaly at least once, maybe twice in my experience.  It was corrected by using the aforementioned procedures.  If that had failed to correct the problem, then certainly the most conservative course of action would be to get on the ground and figure it out there.

Welcome to Av.SE!  Thanks for adding to this thread - real world experience is always a great addition. Please continue contributing here.


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Why is this Jordan–Afghanistan flight avoiding airspaces in the Middle East despite being used by other flights?
Not sure if this particular question fits more on Aviation or Politics.
I am following the international situation these days and sometimes I watch a bit of live Flightradar.
Today en route to Kabul I stumbled upon this anonymous Boeing 737 BBJ that apparently has no registration number, etc, but looks like having departed from Amman to Kabul.

The map shows that the pilot really wanted to avoid certain territories like hell. I know that air routes are (more than) often dictated by political relationships. I also knew that Israeli flights have to take considerable detours when flying over unfriendly countries, especially before the agreements with the UAE.
A direct route involves flying through Iraq and Iran. Clearly, we can all see these days that all flights not to/from Afghanistan are avoiding it, but what about other countries in the Middle East area? I see a lot of traffic over the skies of these territories.
What could bring the pilot so far from the shortest/cheapest route?

Without knowing more about the aircraft, this might be difficult to say. The aircraft type is listed as "737-7HJ(BBJ)", meaning it is a Boeing Business Jet.

So it's not a commercial flight (indeed, these days there should be none inbound). May be a presidential/private superjet, then? But presidential flights are normally identifiable, aren't them?

It's not a commercial flight. It could be a government or international organization (I doubt rich private individuals would travel to Afghanistan in their private jet right now). The fact that FR24 doesn't know about the registration number, doesn't imply the aircraft doesn't have one.

What I want to know is why they avoided Turkmenistan, of all places...

State aircraft - especially Military - do not have the ICAO freedoms of the air. They must apply for diplomatic clearance to transit the airspace of a foreign country. Often this takes days or sometimes weeks to obtain and depending on who you are and where you are going can mean that certain routes may be preferred, especially at short notice.  This may be the reason, if it's a State Aircraft (Govt / Mil), you see this aircraft avoiding certain countries.

Thanks to @Arkhem for suggesting a government flight. Upvote him/her post if you wish, it contains the exact reason for not flying over certain areas, I'm just adding a complement to this answer.

It's possibly Hillwood airways, flight HWA228, using N737AT which ICAO id is A9E51E.
This aircraft was once owned by US DoD, and according to this tweet and this post Hillwood is currently involved in an airlift under a contract with DoD:

N737AT, source
It was flying again today, but disappeared from the ADS-B tracking network soon after passing Cyprus:

Thanks to @ymb1 for suggesting the ADS-B tracking site ADS-B Exchange which allowed me to check for the aircraft last flights.

Just to check, that's Cyprus in the picture, did it turn left and head down the med or did it turn its ADS-B off before heading into Turkey?

@Arkhem: My mistake, this is Cyprus not Crete. The track stopped here, so either there was no close receiver on the ground (unlikely), or the transmitter was turned off.

One good motivation to avoid certain airspaces is presented in the stunt recently pulled by the Belarussian authorities:
Ryanair Flight 4978 was forced to land in Minsk on false pretenses, and two passengers were then taken off the plane.
For a special flight such as the one in question here, it may very well be worth to fly not just a bit of extra to avoid the possibility to get hijacked by a government.
Normal passenger traffic does not have the luxury of catering to every passengers safety needs.

I know that case. But I'd know for sure that if I was a refugee flying from safe country A to safe country B, a lowcost airline would never listen to me speaking at the check in desk "I should advise you not to fly over country C airspace because I have a bounty over my head in C and your flight could be... uhm... delayed". What I really mean is that nowadays I am skeptic that commercial airlines could change their route for a very single name on the manifest. A state/military/private flight is different

Duh, I'm refferring the specific flight in this question, not any tube full of tourists. I thought that would be quite obvious, better  edit... normal passenger flights would not plan routes depending on who's wanted where, of course they wouldn't. Even if a passenger demanded a course change or that the crew disobey order given by, say, a couple of fighter jets, well you see the point. This specific flight, as you can see from the answer by mins has a very good reason to avoid as many middle eastern airspaces as possible


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Does ground speed represent the horizontal speed measured at sea level?
Which of the following vectors is refered to as the Ground Speed?
The green one I expect is TAS, the blue one would be Vertical Rate

Note that for simplicity I did not draw them as geodetic curves, but the black one should be shorter than the gray one because of the earth's curvature.
Does the Ground Speed represent the horizontal speed relative to surface at the sea level?
Ex: Helicopter climbing verticaly will have GS=0?
Does the Earth's curvature affect the GS? As the length of the gray vector will be longer the higher the altitude is. Does that mean a plane flying at 1km will have higher GS than a plane flying the same total speed at 10km?
edit:
I'm simulating aircraft movement along a straight trajectory by constant speed. To simplify, the input is start coordinates & altitude and finish coords & alt.
That means the plane can ascend or descent during simulation. I know the total distance between A & B and distance at sea level. So first thing I tought of was:
GS = distanceAtSL / (totalDistance / speed)

In relation to image:
I know the length of Green line and I can calculate how long it will take the plane to reach the end. The time to finish is constant for each line & I know the length of the black line so using formula speed = distance / time I should be able to get the "Ground Speed".
Correct me if I'm wrong.

The earth's radius is about 6370 km. This means a difference of about 0.1% in distance over ground between the plane at 1km and the plane at 10km.

Well a 2100 km/h at 10km flat is 2096,704227 km/h projected at sea level, according to Geodesy library i'm using. Point is what value would be refered to as the Ground Speed. I forgot its at 50°N 1°E as earth is not sphere

@mins ground level = Sea level. That's what I need. I do not account for wind or pressure, the input speed is simply the speed the plane moves between point A & B including the altitude difference, so GS is simply the projection to ground where the alt difference is not accounted for. Like looking from the orbit right above the plane (Top Down View).

2100 km/h? what planes are you considering?

When you are considering a) absolute velocity vector relative to sea level surface, versus b) component of above that is actually parallel to sea level surface, versus c) --? -- component of above that is actually parallel to instantaneous slope of earth below??- I'm not sure that there really is a universally accepted definition.

Your case is:

An aircraft flies from A to B which 3D coordinates are known
The 3D distance flown, AB, is known
The flight time is known, or can be calculted from the aircraft acutal speed
Wind, temperature, pressure, or any other actual atmosphere element are not taken into account

Your question is:

What is the ground speed?

Short answer: 1/ Yes, the ground speed is the speed on the aircraft trajectory projected at sea level. 2/ Sea level is not an ellipsoid of revolution, but a surface close to the geoid of reference. 3/ Using the geoid geometry is however not advisable. It's complex, requires an appropriate GIS library using a geoid (gravitational) model, needs more time for calculation. Sticking to the ellipsoid is usually a good alternative.

When simplified, your problem is not an aviation problem, only a geometry one.
The ground speed is the speed of the orthogonal projection of the aircraft on the brown arc from A' to B'. We can only look for the mean speed because this ground speed is not constant if the green curve flown is not exactly an homothetic transformation of the brown curve, flown at constant speed.
As you said, you need two parameters: Distance and time and you have already one: Time to fly from A to B.
To compute length A'B' of the brown arc, just project A and B orthogonally onto the Earth surface using your 3D library functions and compute the curved distance.

By Earth surface, I mean what your 3D library allows, e.g. projection on the ellipsoid, or if required and possible, projection on the geoid.

Having now the length of the brown arc A'B' and the time flown, the ground speed is the ratio of the two.

Note: The circumference of a circle around Earth surface at kilometer above it is only 6.28 km (2π) longer than at Earth level, and each km of height adds only this same quantity. So at 10 km, the difference with sea level is (only) 63 km.

The library I'm using uses WGS84 elipsoid model. I can ofc. use the lat/long deltas each simulation step to derive the Ground Speed. It should technically give same results as my method as I use the same library to update plane coordinates each step based on speed, bearing & start point.

Will mark this as an answer for now.

Both the black vector and gray vector. Like land surveying, ground distance is always level. ( "level" curves with the globe, so right angle to any plumb line.) When land surveyors encounter a mountain, angular slope distance is not used, it is effectively stair steps of pure elevation and pure level distance. Keep in mind that the earth is as smooth as a billard ball(57mm), to that scale Mt Everest would be 0.04mm(1.6/1000 inch) and the ovalness of 22km(69,000ft) radius would be 0.1mm(4/1000 inch). Making the error between black and gray at FL350, ~0.084%
More accurately stated, ground speed is angular speed around the earth, approximately converted to an equivalent linear speed at sea level.
The original source of a nautical mile is one minute of a degree of angle around the globe. (So all planets are 21,600 nautical miles in circumference.)
And this is why nautical miles are much easier to use for traditional long-distance navigation using trigonometry. Where an exact position is more important than an exact linearized distance, especially due to the influence of currents/wind on the effective travel distance.

When navigating, you must decide what is more important; position, ground distance, air/water distance, or course angle.(air/water distance between two fixed points is altered by currents.) IOW Do you care that you traveled 5005miles instead of 4095 miles over the ocean, or do you care more that your current position before descending through the clouds is within one mile of the small atoll with a landing strip?


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Best books for learning about history of airplanes?
I've always liked airplanes, but for some reason have never retained much information about specific planes. For instance, if someone casually mentions the X-15 or the X-24, I have to look up what plane they're referring to, and even then I don't know much of their history (with an exception for the Wright Flyer). I'd like to change that and get up to speed with the vernacular and general knowledge of these cool aircraft. Besides the internet, are there any books you would recommend on general aviation history?

Wikipedia and the power of the blue link. I might advise starting with any major manufacture and making use of the "Aircraft Models" section at the bottom of the page.

An old, but good, book on aviation tech is The Lore of Flight.  The publication date is circa 1970s, but it’s a decent primer on all aspects of aircraft technology including, history of aircraft, structures, landing gear, powerplant, mission systems, VTOL flight, armament, and general aviation

That book was on the shelf in the library of my elementary school....